Various attempts have been made in the past to provide adequate lubrication for the bearings of spur gear pumps and motors by bleeding off a portion of the fluid flowing through the apparatus and passing this portion through the shaft bearings. For example, the wear plates next to the gears have been provided with a metering slot extending between the shaft openings of the wear plate, in the zone where the gear teeth intermesh. In such a case, lubricant is forced in parallel via the metering slot through the bearings and then collected and returned to the low pressure side of the apparatus. These long metering slots have the disadvantage that they weaken the seal plate so that high strength, expensive materials must be used. Also, the metering slots or notches are subject to both clogging and erosion which can seriously impair the distribution of lubricant. Moreover, air dissolved in the lubricant tends to be pulled out during flow through the metering notches, which leads to frothing of the lubricant and poor flow through the bearings. Finally, such parallel lubrication of the bearings requires rather large volumes of oil which do not reach the outlet port of the pump, thereby reducing overall efficiency.
Attempts have also been made to direct lubricant flow in series first to one bearing and then to another, to reduce the overall volume of oil required for bearing lubrication. For example, it is known to provide a short metering slot which extends from the zone where the gear teeth intermesh toward only one of the gear shafts, on the high pressure side. In this case, lubricant is forced in series through one bearing via the short metering slot, through a channel in the pump housing to the other bearing, and then returned to the low pressure side. Unfortunately, this prior art design is subject to several of the drawbacks noted previously for the parallel flow pump. Location of the metering slot on the high pressure side requires the use of a carefully sized slot to keep the bearing flow rates within limits, since the high pressure varies under load. Thus, the bearing flow is controlled by the system pressure and is difficult to regulate. Moreover, the metering slot is still subject to clogging and erosion due to its rather small size. As the volume between the gear teeth first decreases and then increase in gear pumps and motors, flow reversals are known to occur in the metering slot which can lead to less desirable lubricant flow patterns and lubricant frothing. Finally, location of the short metering slot on the high pressure side places a large pressure differential on the wear plate which tends to cause increased wear.
Another approach to controlling the lubricant flow through the bearings of a gear pump or motor is disclosed in commonly assigned U.S. Pat. No. 4,160,630, issued to Wynn. This patented device includes wear plates having a flow channel positioned on the low pressure side, the flow channel being exposed to the elevated fluid pressure which exists in the small volumes of fluid caught between the meshed gears of the device. This elevated pressure is used to force lubricant flow through one of the adjacent bearings, along a channel provided in the housing and back through the other bearing on the same idea.
While these small volumes of fluid do experience pressures in excess of the low side pressure over a portion of the gear revolution, it has been found that as the small volumes expand as the gears continue to rotate toward the low pressure side, the pressure of these volumes actually drops below the low pressure for a time. This reduced pressure has been used in some prior art pumps as a means to draw lubricant from one of the adjacent shaft bearings into the small volume from which it escapes to the inlet side of the pump. Simultaneously, lubricant is drawn into other pump bearings. U.S. Pat. Nos. 3,447,472 issued to Hodges et al and 3,490,382 issued to Joyner disclose such systems, as does West German Pat. No. 1,528,959 issued to Weigert.
Although these prior art pump designs have achieved varying degrees of success, seal plate geometries have tended to be rather complex and often the plates have had different geometries on opposite sides of the gears. So, a need has continued to exist for a gear pump or motor in which the seal plates are of considerably simplified geometry which does not require such precise positioning of flow channels or the use of opposite-handed seal plates on the opposite sides of the gears.